Silica long has been used, either per se or coated with an organic material, as a stationary phase in chromatography, and as such has enjoyed broad success and applicability. As new chromatographic needs arose, these often were met by changing the properties of silica, so that several discrete kinds of silica have been used as a stationary phase. For example, Okamoto and his group recently have described chiral stationary phases , i.e., stationary phases coated with a chiral organic material and used in the chromatographic separation of racemic mixtures to afford chiral components, and has found that such separations are effected particularly well using a large-pore silica as a support, i.e., silica having pores on the order of 300-1,000 angstroms. See, for example, Y. Okamoto and Y. Kaida, J. Chromatography A, 666 (1994) 403-419.
Silicas containing large pores are available, but routes to their preparation result in high cost, low reproducibility, and limited availability. For example, a pore-filling/melt procedure has been successfully used to prepare large-pore silica with pore sizes 1,000 angstroms and greater. In this procedure silica is impregnated with a salt, such as sodium chloride, by an incipient wetting technique, so the pores are filled with a solution of sodium chloride. The wetted materials are carefully dried, then heated to a temperature where sodium chloride liquefies and the pore structure of silica is disrupted. Upon cooling, the silica recrystallizes around the salt, generating pores within the bulk silica of large size (.gtoreq.1,000 angstrom pores). Subsequently the salt is washed out to leave silica having the aforementioned large-pore structure.
It has been observed that the silicas prepared by the foregoing method have a bimodal pore distribution. Secondly, we have observed that the resulting silicas have little, if any, silanol functionality, which may be important where silica is used as a support for an organic coating. This observation is reasonable given the high calcination temperatures used. Last, the method is somewhat inconvenient to apply in making commercial sized batches.
What we required was a silica having large pores (at least 300 angstroms) with a narrow pore size distribution, a relatively low surface area (20-30 square meters per gram (m.sup.2 /g)) and particles at least within the 50-150 micron range. It was required of any method of making such materials that the method be readily adaptable to commercial-size runs, that it afford control over pore size so as to give a reproducible pore size and distribution, that it was applicable to silicas generally, and that it was relatively inexpensive and did not require specialized equipment or a severe heat treatment.
What we have found is that when a silica powder is contacted with a mineralizing agent such as sodium hydroxide, in the temperature range of 85-300.degree. C. for a time as short as four hours and up to several days, one introduces into the silica large pores with a unimodal size distribution. This offers a general route to large pore silicas whose pore size can be readily controlled, which can be practiced effectively without specialized equipment, which avoids the high temperatures of prior art methods, and which is readily practiced on a large, commercial scale.
Araya et al., U.S. Pat. No. 5,354,548, prepared a porous silica by forming an oil-in-water emulsion whose oil phase was greater than 50 volume percent from an aqueous solution of a silica precursor and a suitable water-immiscible liquid, and then gelling the continuous aqueous phase of the emulsion. Kislev et al., U.S. Pat. No. 3,888,972, subjected a silica xerogel to hydrothermal treatment at 100-380.degree. C. and 1-100 atmospheres followed by drying at 100-300.degree. C. to obtain silicas with an average pore diameter in the 230-30,000 angstrom range. Xerogels are dried gels with high surface areas (typically 500-900 m.sup.2 /g) with small pore sizes typically in the 10-100 angstrom range. The patentee of U.S. Pat. No. 3,975,293 treated silica gel in an ammoniacal medium under pressure to obtain pores up to about 2000 angstroms. In U.S. Pat. No. 3,869,409 the patentees teach preparation of large pore silicas by preparing an aqueous suspension of silica containing an alkali, alkali or alkaline earth metal salt, or carbon black, then drying the suspension (i.e., evaporating all the water) to form a xerogel with enlarged pores. Acker, U.S. Pat. No. 3,526,603, made intermediate density gels by washing an acid-set silica hydrogel with a hot ammonia solution, neutralizing the base washed hydrogel, washing, and then drying the hydrogel. Finally, U.S. Pat. No. 4,474,824 teaches methods of preparing and treating hydrous silica gels to increase their abrasiveness by contacting the gels with an alkaline medium or with an aqueous acidic solution at temperatures of 80-100.degree. F.(27-38.degree. C.).
A distinguishing feature of our invention which separates it from the foregoing art is our use of a silica powder in the preparation of large pore silicas. In this application "silica powder" means a collection of discrete, solid silica particles with a size (i.e., diameter or long dimension) in the range of 25 microns to about 5 mm. Our use of a silica powder is a glaring departure from the teachings of the prior art, and makes our method of preparing large pore silicas far more general, more convenient, and more susceptible to commercial-scale manufacture than previous approaches. The process which is our invention does not proceed via a hydrogel and does not occur via gelatinization, which is a critical distinction over the prior art. Instead the treated silica powders may be directly separated, washed, dried, and used per se.